For a bipolar transistor, it is desirable that the collector current IC is only a function of the base current IB with the ratio of these two being defined as the current gain of the transistor. However, as shown in FIG. 1, for a constant base current the collector current is not independent of the collector-emitter voltage in the region, generally designated 10, where the transistor is fully operational but instead the collector current varies with the voltage across the transistor. This region 10 can be extrapolated backwards to an intercept with the voltage across the transistor, and the intercept, generally designated VA, is known as the “early voltage.”
It is desirable for the slope of the region 10 to be as near horizontal as possible, which corresponds to a desire for the early voltage to be as large as possible, within the confines of other constraints on the fabrication of the transistor, such as the fabrication process used to form the transistors or at least the majority of the transistors within a semiconductor substrate.